Respiratory syncytial virus (RSV) is a highly infectious seasonal respiratory disease and is a significant cause of morbidity among young children[1]. It is the leading cause of hospitalisations for bronchiolitis and pneumonia among Australian infants aged < 2 years, with greatest severity observed in early infancy[1, 2]. Approximately half of all paediatric RSV-related hospital and ICU admissions in Australia are among infants < 6 months of age[2]. Birth shortly before or during RSV season, household crowding, living with siblings, daycare attendance and socioeconomic disadvantage are frequently cited as factors associated with higher risk of RSV infections[3–5].

Timing of seasonal RSV epidemics varies between Australia’s climate zones[6, 7]. RSV primarily circulates during winter months in temperate regions[7, 8], while tropical areas may experience year-round circulation or monsoonal outbreaks[6, 7]. In contrast, epidemic dynamics in arid zones are poorly understood. Climate change is expected to alter regional epidemic dynamics over time[9].

In 2023−2024, the Therapeutic Goods Administration (TGA) approved two novel preventative agents to reduce RSV incidence and severity in young infants[10, 11]. These include a maternal RSV vaccine offered during the third trimester of pregnancy (Abrysvo), and long-acting RSV-specific monoclonal antibodies for infants (nirsevimab)[12]. Since 01 February 2024, Queensland Health has provided year-round access to a free birth-dose of nirsevimab for all infants. This is available up until 8 months of age, or < 2 years in infants with additional risk factors for severe RSV disease[13]. All states and territories now provide similar programs with varying eligibility requirements, and year-round or seasonal availability (April−September)[12, 14]. Jurisdictional nirsevimab programs form part of the national RSV Mother and Infant Protection Program (RSV-MIPP), which since February 2025 has additionally provided free access to RSV vaccination for all pregnant women under the National Immunisation Program (NIP)[12, 14].

RSV was designated a nationally notifiable condition in July 2021[15]. With nirsevimab introduced in Queensland in February 2024, the years 2022−2023 represent the only complete calendar years of RSV notification data available prior to the introduction of RSV immunisations. Understanding the epidemiology, community-level determinants, and spatiotemporal dynamics of RSV incidence among Queensland infants prior to the introduction of the RSV-MIPP is critical for evaluation of these funded programs, and for achieving optimal and equitable state-wide coverage.

The objectives of this study were to quantify pre-program incidence of RSV notifications among Queensland’s infants aged < 2 years, describe spatiotemporal differences in this incidence between Queensland’s diverse climate zones, and to identify high-incidence spatial clusters and community-level determinants of incidence.

For this retrospective observational study with spatial analysis, we included all infants aged < 2yrs residing in Queensland between 01 January 2022 and 31 December 2023 inclusive (n = 113,555). RSV became a nationally notifiable condition in Australia in July 2021, which falls during the annual RSV season for most of the southern hemisphere[8, 15]. This study period was chosen to capture two complete RSV seasons. Aligned with Queensland Health RSV notification data practices, consecutive RSV notifications for the same infant were counted as discrete infections if they occurred more than180 days apart.

Population estimates for incidence calculations were obtained from 2021 Australian Bureau of Statistics (ABS) census data[19]. For spatial analysis, census population estimates were obtained by single year of age (0 and 1 year) and postcode area.

Climate zones were determined using the Australian Building Code Board (ABCB) climate zone designation by postcode area[22]. ABCB climate zones are derived from Australian Bureau of Meteorology climate zones, which describe both temperature and humidity[22, 23]. Four out of the total eight Australian climate zones are present in Queensland. Zone 1: high humidity summer, warm winter (tropical); Zone 2: warm humid summer, mild winter (subtropical); Zone 3: hot dry summer, warm winter (arid/semi-arid); and Zone 5: warm temperate[22].

Incidence of RSV notifications were calculated per 1,000 total Queensland population, by 5-year age group and year of notification, then by each completed month of age for infants aged < 2 years. Epidemic curves were constructed to demonstrate weekly incidence among infants, by climate zone of residential postcode and notification year. Poisson regression (generalized linear model) was used to calculate the relative risk (RR) of RSV notifications among infants by postcode and climate zone (index: zone 1, high humidity summer, warm winter), IRSAD quintile (for each one-quintile reduction in advantage) and ARIA designation (for each one-quintile increase in remoteness). Adjusted RRs (aRRs) were subsequently calculated including all covariates. Results are presented as RRs/aRRs with corresponding 95% confidence intervals (95% CI).

We applied Kulldorff’s spatial scan statistic (Poisson model) to detect high-incidence spatial clusters of observed vs expected RSV notifications by year. P-values were obtained through Monte Carlo hypothesis testing (99,999 iterations), and clusters deemed as significant where p < 0.05.

Data were cleaned and analysed using the statistical software package StataSE v.18, and graphs and figures were constructed using StataSE v.18 and Microsoft Excel. Spatial clustering analysis was conducted using SaTScan v10.2.1 and maps developed using ArcGIS v10.8.2. SaTScan outputs do not provide 95% CIs.

Among infants aged < 2 years, incidence of RSV was persistently high from 1–15 months of age. Highest incidence was recorded among 1-month-olds and 12-month-olds, the latter of whom are currently ineligible for protection with nirsevimab under the RSV-MIPP unless they have additional risk factors for severe disease. We identified high-incidence spatial clusters of RSV notifications in both major population centres and regional areas and found that remoteness of residence and number of children per family household may be important community-level determinants of RSV incidence. The role of community-level socioeconomic disadvantage in determining incidence was less clear, possibly due to unmeasured confounding from socioeconomic differences in utilisation of healthcare, childcare, and other services. RSV incidence varied between Queensland’s climate zones, with greater incidence recorded in lower humidity areas with a more distinct winter season. Timing and magnitude of RSV epidemics among infants also differed between Queensland’s climate zones, highlighting differences in key risk periods for RSV transmission across the state.

Given the frequency of severe outcomes among infants[1, 2] and compelling evidence of a casual association with the onset of childhood asthma following RSV infections in the first year of life[2, 24], high incidence of RSV among infants represents a major public health issue. The RSV-MIPP is likely to reduce incidence and severity among Australia’s youngest infants, however we observed persistently high RSV incidence from birth until 15 months of age. Maternal RSV vaccination and nirsevimab administered at birth have each been demonstrated to protect infants until approximately 6 months of age[25, 26]. Older infants > 6 months of age are unlikely to be adequately protected by current interventions. Options to extend the duration of protection against RSV into later infancy should be explored.

Family size, household crowding, and RSV transmission dynamics are important considerations in future program planning. Household transmission between siblings is a major driver of RSV incidence, with children who are exposed to RSV at school or daycare subsequently spreading infections to other children in their home[4, 27]. In light of this, nirsevimab and maternal RSV vaccination may be particularly cost-effective when used to protect infants who live with older preschool or school-aged siblings for whom RSV immunisation is not yet available[12]. Childcare centres may represent an effective setting to raise awareness of RSV and its prevention among parents.

The effect of community-level socioeconomic disadvantage on RSV incidence among infants was not statistically significant in the adjusted model, and increasing remoteness appeared to be associated with lower RSV incidence. Considering that remote and disadvantaged communities in Australia are often burdened by established risk factors for RSV transmission such as household crowding[28], these results should be interpreted cautiously. Our findings may be explained by lower rates of RSV testing in more socially disadvantaged or remote regions, smaller populations in remote areas, or residual effects of the COVID-19 pandemic. It is also plausible that remoteness may be associated with decreased incidence of RSV among infants due to lower and/or later initiation of attendance at formal childcare services in rural and remote areas of Queensland, where service availability is often limited[29]. Similarly, the high cost of centre-based childcare in Australia is associated with reduced attendance among children from socioeconomically disadvantaged households[29]. Future studies may elucidate whether access to local childcare services is an independent spatial determinant of RSV notifications. Despite our observations, it is well established that socioeconomically disadvantaged and remote residents experience significant cost, availability, distance, and cultural barriers to accessing health care services in Australia[30]. Prioritising equity in new RSV prevention programs is critical to ensure optimal coverage among all eligible infants.

Queensland’s tropical and subtropical zones demonstrated year-round circulation of RSV with lower-incidence peaks, which occurred earlier in 2023 compared to other zones. Previous studies have found peaks occurring during periods of high rainfall in North Queensland and other tropical regions[6, 8]; with lower overall incidence thought to be determined by a combination of consistent high humidity and lack of a distinctly cooler winter season[6]. Findings were similar for subtropical areas, albeit with a more distinct seasonal peak in 2022 than observed in the tropics. Whilst overall incidence was lower in the tropical and subtropical zones compared to other parts of Queensland, significant clusters were still identified in several densely populated areas within these zones.

Epidemics in warm temperate (zone 5) and arid/semi-arid (zone 3) regions with distinct winter seasons were more discrete, with consistent high-incidence seasonal peaks observed in zone 5 across 2022−2023. Incidence in the arid/semi-arid interior regions of Queensland was high in 2022, but this was not repeated in 2023. There is some evidence that RSV outbreaks may occur biennially in some non-tropical regions of Australia[7], and future years of RSV notification data will clarify whether this is true for Queensland’s dry interior. Understanding these patterns will be necessary for targeted vaccination program planning and rollout.

The introduction of the RSV-MIPP is likely to reduce incidence of RSV infections and severe outcomes among younger infants, however older infants are unlikely to be adequately protected. Protective strategies for these infants warrant further attention. We found higher incidence of RSV notifications in 2022−2023 in areas with larger family sizes, and in areas with distinctly cooler and lower humidity winters. Lower RSV incidence observed in remote areas may be attributable to lower rates of testing, or limited access to childcare services. Community-level determinants of RSV incidence should be considered in RSV-MIPP evaluation and planning.